Stormwater pollutants caused by urban runoff are a major concern for the Albuquerque Metropolitan Arroyo Flood Control Authority (AMAFCA), and other flood control authorities, as stormwater channels eventually are discharged into natural waterways. One method of improving stormwater quality is diverting frequent storms or the first flush of less frequent storms to a structural debris filter and/or a constructed wetland prior to reentrance into the channel. The focus of this study was to develop a procedure to allow determination of laterally diverted flows of stormwater from a supercritical channel for treatment.
A range of design flows were tested and evaluated for a trapezoidal channel with a lateral outflow pipe. Lateral pipe angle, pipe invert, and vane configurations were tested for outflow efficiency and to observe hydraulic effects in the main channel. A bench-top model was constructed and tested to determine which pipe invert, configuration, and pipe diameter would be used for the larger scale experiments.
An 8 inch (20.32 cm) diameter pipe set across the base of the channel with a pipe invert of 50% increased the diversion ratio within the range of 15% to 100%, depending on the configuration tested. Three pipe angles, 30°, 45° and 90°, were used in the experiment with the 30° pipe diverting the highest amount of water, with 97.8% being the lowest at a Froude number of 2.71 and velocity of 2.97 ft/s (0.905 m/s), with little to no negative hydraulic effects to channel flow. A uniform and tapered vane structure at different vane angles, 35°, 40° and 45°, was added to the different pipe angles to increase the diversion ratio. A tapered vane at a 45° vane angle diverted the most water, with a diversion ratio of 97.31% being the lowest at a Froude number of 2.96 and a velocity of 3.76 ft/s (1.15 m/s), with a stable jump upstream of the vane.
HEC-RAS was used to model three different pipe angles relative to the main channel axis to determine if the numerical program was able to capture the hydraulic effects observed from the physical model. The results from the program are in agreement of the adverse affects that were seen at the different pipe angles. The program was able to capture the adverse hydraulic effects, rolling waves and hydraulics jumps, in the physical model with similar upstream velocity and Froude number values.

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